A moving object responds very slowly to gravity
Gravity forces

You need to be aware that gravity is a force. This force will be applied to the current linear velocity.

The linear velocity gets increased over time as you add the force again and again, resulting in a larger linear velocity.

Therefore you get:

linear velocity = previous linear velocity + velocity from gravity


which is:

linear velocity = initial velocity + velocity from gravity + velocity from gravity + ... 


Situation

I assume you explicitly set the linear velocity on key press. 

In this case the final linear velocity will not add up as you already reset it as reaction on the key press.

The result is:

linear velocity = motion velocity + velocity from gravity


The gravity can't simply add up.

Possible Solutions

Motion as force

When you apply the motion as force your motion force will not override the previous linear velocity. It will be added the same way as the gravity force will be added.

This allows smooth but unlimited acceleration.

Servo Control

The motion mode "Servo Control" allows to setup a linear velocity without overriding certain axis. I suggest to enable the [Z] button and leave the limits at 0.0 as they initially are.

This allows the object to fall down without accelerating into the direction you want it to move.